The overall objective of this study is to characterize in vivo the capacity of IgE antibodies (in the absence of other classes of antibodies to a given antigen) to induce alterations in pulmonary and circulatory structure and function and to elucidate the mechanisms by which such changes occur. Many studies of other investigators have been performed to established potential mechanisms of IgE-induced allergic reactions using in vitro techniques. It is our intention to determine the relative contributions of such potential mechanisms in the in vivo response. Our basic experimental protocol involves manipulation of an animal model we have previously developed in which rabbits can be made to produce IgE antibody (in the absence of other classes of antibody) to soluble protein antigens (J. All. Clin. Immun. 49:301, 1972). Thus, structural and functional alterations which occur as a result of exposure to antigen can be directly attributed to the IgE class of immunoglobulin. These rabbits undergo systemic anaphylaxis (SA) when intravenously challenged with antigen. Physiological alterations which accompany SA include a rise in pulmonary arterial pressure, a fall in aortic pressure, a transient period of apnea, an increase in total pulmonary resistance, and a decrease in dynamic compliance. In the present study, various manipulations and interventions will be examined in relation to their capacity to mimic or block these physiologic alterations. These include monitoring the effects of mediators and their inhibitors, prior platelet depletion, and anti IgE antibody. Also we propose to obtain rabbit purified IgE and purified sensitized basophils and determine whether giving these preparations to normal rabbits will lead to typical anaphylactic reactions in normal rabbits.